The invention relates generally to electromagnetic interference (EMI) shielding in electronic systems.
The operation of electronic circuitry used in many electronic devices is often accompanied by unwanted stray electromagnetic radiation. Stray electromagnetic radiation or xe2x80x9cnoisexe2x80x9d can interfere with the performance of surrounding devices. Consequently, it is important to shield electronic devices to reduce electronic noise emanating from those devices.
Redundant arrays of inexpensive or independent storage devices (RAID) are being employed by the mass storage industry to provide variable capacity storage. RAID systems use interconnected disk drives to achieve the desired capacity of mass storage. With this approach, a disk drive of one capacity may be manufactured and packaged with the same or different capacity drives to provide the required storage capacity. RAID systems eliminate the need to manufacture disk drives individually designed to meet specific storage requirements. Each disk drive in a RAID system is usually housed in an individual module for handling and installation. The modules slide into and out of a larger enclosure that houses the array of disk drives and provides the sockets, plug-ins and other connections for the electrical interconnection of the drives. Controllers orchestrate the interconnection and control access to selected disk drives for data reading and writing operations.
Each module includes a plastic housing and, in most cases, some type of metal EMI shielding. Metal shielding is often constructed as metal plates, panels, partial enclosures and the like positioned within or about the housing. The metal attenuates stray electronic signals emanating from the module as well as stray signals coming from surrounding modules. The degree of attenuation increases with the amount, placement and composition of metal shielding. A closed metal box, for example, would provide excellent shielding. The housing, however, must also permit sufficient air flow to cool the device during operation. Hence, there must be adequate openings in the housing and the shielding to provide the necessary degree of cooling air flow.
Air flow openings in EMI shields are typically round, square or hexagonal. As the frequencies of the electromagnetic radiation/noise are increased, the size of the openings are decreased as much as possible while increasing the length of the openings to provide waveguides that attenuate as much of the electromagnetic radiation as possible while still allowing sufficient cooling air flow. The shape of these waveguides stays the same across the length of the opening. A conventional waveguide for EMI shielding, designated by reference number 2, is shown in FIGS. 1 and 2. The range of frequencies of electromagnetic radiation that will be received into and attenuated by the waveguide is determined by the shape of the opening.
In one embodiment, the invention is directed to an EMI shield that utilizes a waveguide having a three dimensionally non-uniform opening. In another embodiment, the invention is directed to an EMI shield that utilizes a waveguide structure in which the entry, and preferably the exit, is substantially larger than the elongated conduit portion of the waveguide that extends between the entry and the exit. In another embodiment of the invention, each waveguide opening is characterized by a weak receiving antenna, and preferably a weak transmitting antenna, and a waveguide operatively connected between the receiving and transmitting antennas.